In a constant velocity universal joint, which is used to construct a power transmission system for automobiles and various industrial machines, two shafts on a driving side and a driven side are coupled to each other to allow torque transmission therebetween, and rotational torque can be transmitted at a constant velocity even when each of the two shafts forms an operating angle. The constant velocity universal joint is roughly classified into a fixed type constant velocity universal joint that allows only angular displacement, and a plunging type constant velocity universal joint that allows both the angular displacement and axial displacement. In a drive shaft configured to transmit power from an engine of an automobile to a driving wheel, for example, the plunging type constant velocity universal joint is used on a differential side (inboard side), and the fixed type constant velocity universal joint is used on a driving wheel side (outboard side).
Irrespective of the plunging type and the fixed type, the constant velocity universal joint includes, as a main component, an outer joint member including a cup section having track grooves formed in an inner peripheral surface thereof and engageable with torque transmitting elements, and a shaft section that extends from a bottom portion of the cup section in an axial direction. In many cases, the outer joint member is constructed by integrally forming the cup section and the shaft section by subjecting a rod-like solid blank (bar material) to plastic working such as forging and ironing or processing such as cutting work, heat treatment, and grinding.
Incidentally, as the outer joint member, an outer joint member including a long shaft section (long stem) may sometimes be used. In order to equalize lengths of a right part and a left part of an intermediate shaft, the long stem is used for an outer joint member on the inboard side that corresponds to one side of the drive shaft. The long stem is rotatably supported by a rolling bearing. Although varied depending on vehicle types, the length of the long stem section is approximately from 300 mm to 400 mm in general. In the outer joint member, the long shaft section causes difficulty in integrally forming the cup section and the shaft section with high accuracy. Therefore, there is known an outer joint member in which the cup section and the shaft section are formed as separate members, and both the members are joined through friction press-contact. Such a friction welding technology is described in, for example, Patent Document 1.
An overview of the friction welding technology for the outer joint member described in Patent Document 1 is described with reference to FIG. 29 and FIG. 30. An intermediate product 71′ of an outer joint member 71 includes a cup member 72 and a shaft member 73, which are joined through the friction welding. As illustrated in FIG. 29, burrs 75 are generated in at least one of inner and outer diameter portions on a joining portion 74 along with the friction welding. In order to mount a rolling bearing (see FIG. 1) to a shaft section of the intermediate product 71′ of the outer joint member 71, as illustrated in FIG. 30, it is necessary to remove the burrs 75 on the radially outer side of the joining portion 74 through processing such as turning. Although illustration is omitted, the intermediate product 71′ is processed into a finished product of the outer joint member 71 through machining of a spline, snap ring grooves, and the like, and through heat treatment, grinding, and the like. Therefore, the outer joint member 71 and the intermediate product 71′ have slight differences in shape, but illustration of the slight differences in shape is omitted in FIG. 30 to simplify the description, and the outer joint member 71 being the finished product and the intermediate product 71′ are denoted by the reference symbols at the same parts. The same applies to the description below.